The invention relates to improved cyclone separators for removing particulate material from fluids including a wide variety of liquids, gases, and more particularly, relates to improvements in such separators for controlling boundary layer turbulence in the vortex flow of fluid therethrough in order to prevent particulate material from inducing turbulence in the boundary layer and causing reentrainment of particulate material in the vortex flow. The basic principles common to most cyclone separators have been known and used for years. Such separators have been successfully used to purify both liquids and gases by separating entrained particulate matter from them. In addition, cyclone separators have been used to separate both liquids of differing viscosity, and entrained particles of differing sizes, from one another. For example, such devices are useful to separate cream from milk, entrained water from steam, as well as to separate dust from air supplies and smoke from the discharge gases in chimney flues.
In general, cyclone separators operate by subjecting a flow of fluid and entrained particles passing through the separator to a combination of forces that are effective to drive most of the particules to an area of the separator where the particles can be readily extracted from the fluid flow. Typically, a cyclone separator includes a generally cylindrical chamber for receiving inlet fluids, and a frusto-conical section for establishing a vortex flow of the fluid toward the truncated apex end of the conical section. Fluid is introduced under high pressure tangentially into the upper end of the cylindrical chamber and flows helically into the frusto-conical section which establishes a vortex flow in the fluid to increase the centrifugal forces and angular velocity gradient applied to the fluid flow and the entrained particles. In addition to those forces, a pressure differential exists in the separator from the inlet end toward the outlet end thereof as well as from the periphery of the cylindrical chamber and frusto-conical section toward the central longitudinal axis of the separator.
The centrifugal force exerted by the toroidal flow of the fluid passing through the separator acts to propel the heavier matter in the fluid flow toward the circumference of the separator walls. Further complex forces operate to separate heavier particles from the fluid flow passing through such separators. Normally, the longitudinal axis of a cyclone separator is veritically oriented so that the force of gravity is effective to more heavy particles in the vortex fluid flow toward the outlet of the separator at the truncated end of its frusto-conical section. There also exists an hydraulic drag force or effect on particles in the vortex flow through cyclone separators. This hydraulic drag effect is most pronounced on particles having relatively long bodies that subject different areas of the particles simultaneously to the friction force of the relatively slower velocity fluid near the center of the cyclone separator, while the outer end of the particles are subjected to another friction force exerted by faster moving fluid nearer the periphery of the vortex flow. Such hydraulic drag effects cause longer particles in the fluid flow to tend to move radially inward toward the longitudinal axis of the separator.
In addition to these complex forces present in cyclone separators, there is always a considerable degree of turbulence present in the vortex flow of fluid through a separator. Such turbulence is normally very counter-productive and frequently causes mixing of pariculate matter with the fluid flow, rather than facilitating separation of particulate matter from the fluid, as in normally desired. Thus, turbulence in the vortex flow of a separator has long been recognized as a disadvantage and various measures have been developed to eliminate or diminish such turbulence. One significant source of turbulence in cyclone separators is the introduction of eddy currents into the flow of liquid entering the separator at high velocity. In general, such eddy current turbulence increases as the velocity of inlet fluids is increased in an effort to improve the separating effect of the centrifugal forces acting in the separator on entrained particles.
Various inlet control structures have been proposed to cope with such fluid flow inlet turbulence, as explained for example in a report prepared for the U.S. Department of Energy under its Contract Number ET-78-C-01-3242. The report is entitled, "Laminar Flow Cyclone Development Program" (Quarterly Technical Progress Report, Oct. 1-Dec. 31, 1978) bearing code number FE-3242-2, and authored by W. B. Giles and K. L. Bekofske. Beyond the inlet flow turbulence created by eddy current effects, other causes of turbulence exist within most cyclone separators. The disadvantageous reentrainment effect of such turbulence is particularly noticeable in reducing the operating efficiency of very high pressure cyclone separators that are used to remove very fine particulate material from fluids. One application of such high pressure separators that has grown extensively in recent years is in the field of coal gasification where very fine particulate matter must be removed from coal gas before it is introduced into a power generating turbine. In such applications it is desirable to remove not only relatively large entrained particulate material from the fluid flow, but to also extract as much of the very fine particulate matter, such as particles of one to ten microns in length, as possible. Fluid bed combustion of coal results in entrainment of a wide range of different sized particles in the exhaust gases, so it is necessary to improve their extraction in order to reduce the erosive and corrosive effects that such particulate material has on the high speed gas turbine blades through which the cyclone-purified gas is passed in a typical co-generation process. One known method for removing particulate, in the range of two to five microns, from a gas stream that is to be fed into a gas turbine is to expose the gas stream to laser illumination in order to destroy the particulate. U.S. Pat. No. 4,226,369-Botts, which issued Oct. 7, 1980, disclosed such a process.
Although the effectiveness of cyclone separators can be improved by increasing the velocity of vortex liquid flow through them, it has been found that in prior art cyclone separators the effectiveness of such high velocities is particularly limited relative to the desired extraction of very fine particulate matter, such as that having dimensions in the one to ten micron range. It has been discovered that in such high velocity separators of prior art design, boundary layer turbulence occurs which causes reentrainment of fine particulate material in the vortex fluid flow. Such reentrainment allows the fine particulate material to move radially inward as the vortex flow approaches the primary collector outlet, thus, the fine particulate matter is not extracted from the particulate collector discharge end of the separator, but rather is entrained in the central vortex flow of liquid that moves upward in the separator to the main fluid discharge outlet. Of course, such reentrainment of particulate matter is disadvantageous and it would be very desirable to prevent the occurrence of reentrainment, if possible, so that the discharge fluid from the separator could be optimumly purified.
U.S. Pat. No. 4,231,771-Winsche, which issued Nov. 4, 1980, discloses the use of an inertial separator having a rotary partition means that divides the separator housing into a plurality of axially-extending passageways arranged in parallel. The separator is operable to remove particulate in the range of five to ten microns in size from combustion gases produced by burning coal, thereby to prepare the gases for introduction to a gas turbine. Although such inertial separators are not directly related to the type of cyclone separator disclosed herein, this prior art patent is representative of the efforts that others have made to develop an economically feasible means of dealing with the problem of removing very fine particulate from high volume gas flows in order to reduce the deleterious effects of such particulate on down-stream apparatus, such as gas turbine blades into which such combustion gases are often fed.
The study of boundary layer turbulence control as applied to the flow of fluids over foil surfaces, such as those used in airplanes, is generally quite well advanced. It has been shown that boundary layer turbulence can cause the formation of bubbles or a separating effect between a laminar flow of fluid and the surface of an air foil past which the fluid is flowing. Such a separation of the laminar flow of fluid from an air foil can collapse the lifting effect that the fluid normally has on the foil, it is necessary to prevent the separation in foil applications, such as those on airplanes, where the lifting effect is necessary to support the plane in flight. Early studies demonstrated that laminar flow of fluid over an air foil can be improved by applying suction pressure to the surface of the foil in those areas where boundary layer separation is most likely to occur. Many different types of suction apparatus have been applied to air foils, including the provision of both widely spaced and closely spaced apertures in the lifting surface of the foils, through which suction pressure is applied. In some tests porous plugs have been used in such suction apertures in foil surfaces to control the direction at which fluid is sucked from the foil surface. In other structures, suction air is first passed through a series of fine slots that are cut into the wing skin surface with a diamond slitting saw or other suitable cutting means, then the suction air is drawn through small grooves under the slits and sent into drilled holes through thicker inner sheeting material of the wing and finally into associated suction duct apparatus.
As far as the present inventors known there is little published research data to explain necessary design requirements for the successful application of boundary layer control means to foil surfaces. More particularly, with respect to the field of the subject invention, the present inventors are unaware of any use of boundary layer turbulence control means being applied to cyclone separators, prior to their invention, for the purpose of improving the separating efficiency of such separators. Moreover, it is not seen that either boundary layer suction or pressure regulating methods for use in controlling air foil lifting effect principles, or the disadvantages inherent in boundary layer flow separation from such air foils, finds any readily apparent application in the construction or use of cyclone separators. Recently issued U.S. Pat. No. 4,407,205-Beaufrere (issued Oct. 4, 1983), discloses a coal combustor/gasifier that includes an inertial separator for removing ash from a combustion chamber. The helical flow of gas through the combustor is improved by boundary layer control means somewhat analogous to those disclosed herein.